Metal transmission lines are an increasingly serious bottleneck to the operation of computers, switch-routers, and other information handling systems. The speed of processors and logic has been increasing much faster than the available wiring bandwidth, resulting in unfavorable tradeoffs of power consumption, complexity, and transmission speed. Hence there is increasing interest in using optical interconnection between boards in computers, and even between chips on a board or a multi-chip module (MCM).
Guided-wave optics replaces electrical conduction in wires, with light propagating in dielectric optical waveguides. A dielectric guide is typically a cylindrical structure (often, not necessarily, circular), made up of annular layers. The innermost cylinder is the core, and is surrounded by one or more cladding layers, having refractive indices that are lower than that of the core. Within certain ranges of frequency, position, and angle, light traveling axially in such a structure is guided, that is, it shows no tendency to spread out radially with distance as ordinary light beams do. There is always at least one guided mode in any structure where the core index exceeds the cladding index (two, if polarization is taken into account). A waveguide in which only one such mode can propagate at a given frequency is called a single-mode waveguide. Depending on the waveguide dimensions and refractive indices, there may be other modes as well, in which case the guide is said to be a multimode waveguide.
The optical mode field falls off exponentially with radial distance in the cladding region, and its decay constant depends on the difference between the core and cladding refractive indices. In order to allow the core to be relatively large (several wavelengths in diameter) and reduce losses due to roughness and index non-uniformity at the core-cladding boundary, the core and cladding indices are usually chosen to be close to each other (1%–5% difference), which makes the decay constant in the cladding region a very sensitive function of the index difference. These guided-wave optical devices have many applications, of which the most important is long-haul fiber optic communication.
Modulated optical beams travel almost exactly the same way as unmodulated ones, regardless of how fast the modulation is; there is no tendency for higher modulation frequencies to be lost, in sharp contrast with the high frequencies in a metal wired connection. Guided wave optics is thus a natural candidate for such connections, but the set of requirements inside a computer are quite different from those that are familiar from long-haul communications. The hardware required is different as well. Inside the computer, many parallel connections are needed, each with a single unidirectional data stream coming from one modulated source per line to one detector per line. There are also important applications for one-to-many connections, where one source drives many inputs. These sources and detectors must be inexpensive, because there are so many required (hundreds for a typical server processor board). On the other hand, the very low loss of fibers (0.15–0.5 dB/km) is unnecessary, and no wavelength-division multiplexing is required. Thus, optical interconnections inside computers are likely to rely on polymer waveguides arranged in ribbons, and mass-produced especially for the purpose.
Furthermore, these parallel connections must be remakeable, so that the board can be unplugged for service and a spare plugged in, without requiring that the optical links be replaced. Multimode waveguides and fibers are currently the most common, since their comparatively large cores allow them to be spliced and coupled with loose mechanical tolerances compared to those of ordinary single-mode fibers, as used in telecommunications. Loose tolerances translate into low cost and usually into mechanical robustness as well.
Single-mode waveguides have significant advantages over multimode waveguides, especially in their small diameter and the resulting high interconnection density. Since light in a single-mode waveguide has a well defined guide wavelength, single-mode waveguides allow the use of long gratings and directional coupler structures that rely on a well-defined guide wavelength and field distribution. Hence a remakeable, single-mode, multi-waveguide connector that allowed loose mechanical tolerances would be a significant advance in optical interconnection technology.